a. Field of the Invention
This invention relates to a sizing agent for carbon fibers. More particularly, this invention relates to the sized carbon fibers and a carbon fiber composite material made therefrom.
b. Prior Art
Heretofore, it has been suggested to provide a material having high quality such as light weight, high strength and high modulus of elasticity in various industrial fields--motor car industry, airplane industry etc. Recently, an excellent quality of the carbon fiber was developed and the range of application of the carbon fiber was expanded to sports goods or building materials. These applications for the various fields have been primarily used as composite materials of carbon fiber within a matrix resin of a thermosetting resin (e.g., an epoxy resin, an unsaturated polyester resin, a phenol resin, a diarylphthalate resin), a thermoplastic resin (e.g., a polyamide resin, a polyolefin resin, a polyacetal resin, a polycarbonate resin and a linear polyester resin etc.) and so on.
In this case, it is known to use a sizing agent in manufacturing the composite material in order to make a handling of the carbon fiber easy. Moreover, the importance of the selection of the sizing agent is also known to improve the adhesive property of the carbon fiber to the matrix resin, which depends on the surface treatment of the carbon fiber.
Namely, the adhesive property of the carbon fiber to the matrix resin changes, in general, depending on the surface treating agent used for the carbon fiber. Therefore, in a case using an epoxy resin as a matrix resin, for instance, it is preferred to use an epoxy type sizing agent for the carbon fiber to improve it's adhesive property against the epoxy resin.
However, a reduction of production cycle for composite was required, recently, in order to reduce production costs. In such circumstances, composites of carbon fiber with the unsaturated polyester resin (UP) or vinylester resin (VE) came to be important because of the short hardening time of these resins.
In this case, there was a difficulty with conventional methods using the epoxy type sizing agent, since the adhesive property of the treated carbon fiber against both of the unsaturated polyester resin and the vinylester resin is not good, therefore a shearing strength (ILSS) of inter layers of the matrix resin and the carbon fiber is low--the shearing strength can be a standard of the adhesive property.
Sizing agents overcoming some of these defects are disclosed in the Japan Patent Publication No. 43298/'84 but they have not sufficient ILSS.
Generally, the sizing agent is dissolved in a solvent or dispersed into an emulsion, then it is applied to the fiber by means of dipping etc., and the fiber is dried. In time, the solvent or dispersion medium evaporates then the sizing agent itself fuses to cover a surface of the fiber and a thin film of the sizing agent is formed by which the adhesion of the fiber is improved. Therefore, the sizing agent which can adhere to the fiber enough to strengthen an adhesive property of the fiber to a matrix resin is preferable.
The present inventors had discovered that an unsaturated urethane compound having at least one reactive double bond can satisfy the above required condition and be able to used as the sizing agent when an unsaturated polyester or a vinylester is used as the matrix resin.